Latitudinal Treeline

A couple of days ago, I canvassed the ITRDB data bank for all tree ring series with values later than 1998, noting that there were many new series. Mann has justified the reliance on proxies not updated since 1980 and earlier, and thus not calibrated against recent warmth, on the basis that it is very expensive and time-consuming to do this – a justification which I believe to be laughable on its face. Since white spruce (PCGL) was used in treeline studies in the past, I did a quick collation of new PCGL series as a species identified previously as a temperature proxy to see whether ring widths for new contributions to the ITRDB data bank reflect recent warmth, as they should if tree ring widths were linearly correlated with temperature, as Mann and others have hypothesized.

I didn’t do any sub-analysis to see if the specific white spruce sites were “temperature sensitive” for which Mike Pisaric and Rob Wilson criticized me. My response – and that of other readers of those threads – was primarily to point out that this is what was done in MBH and other studies, which Pisaric and Wilson had never spoken against. Realistically, they aren’t going to speak out against Mann’s use of precipitation proxies in a temperature study so there’s not much point belaboring the matter further.

So let’s go back and re-visit their criticism of how I presented the information. They said that the PCGL series used in my average should not be used in an average because they are not “temperature” proxies but moisture proxies, as anyone can tell by looking at their location, and thus the average was meaningless. As an exercise, I’ve plotted the location of the PCGL proxies with post-1998 values in the map below, colored according to 5 author groups. I’ve also marked ( with a + sign) the 11 “treeline” sites of the original Jacoby and D’Arrigo 1989 study and joined all except the Gaspé site with a line (the Gaspé site is labelled both because it is anomalously located for both the Jacoby and D’Arrigo 1989 “treeline” study and heavily weighted in MBH98. There are many new Jacoby-D’Arrigo sites that have not been archived.

The Meko sites are shown in blue and, as Mike Pisaric says, are well away from latitudinal tree line. I’ve marked with smaller dots the location of all sites in the North American ITRDB data set (actually they go even further south), so you see that a PC1 taken from the MBH98 dataset includes almost no latitudinal treeline sites (and none earlier than about 1500). The Wilmking sites along the Brooks Range, Alaska look as though they must be pretty close to the latitudinal treeline, despite Rob Wilson’s caveats against Wilmking data. Sites in Alaska and along the Rockies may also be taken at altitudinal treelines so this has to be kept in mind when one looks at the map.

My objective was to see what relevant fresh data showed. Looking at this map, my conclusion is that the new Wilmking and Lloyd data from the Brooks Range, Alaska and the Jacoby site in Labrador are the only fresh data that are close to the latitudinal tree line and, accordingly, that examining this particular subset is the best implementation of the implicit Wilson-Pisaric criteria that can be done under current circumstances. There is a considerable amount of presumably relevant Jacoby-D’Arrigo data which is unarchived. But as long as it is unarchived, there’s not much that third parties can say about (and, in my opinion, until the data is archived, the study should not be cited by IPCC or others.)

28 Comments

There is only one archived chronology at ITRDB that is close to latitudinal treeline ak085.crn from Andrea Lloyd. This series goes to 2005, and, I have trouble discerning a strong positive response to recent temperature increases, although doubtless someone will explain to me why this analysis is “flawed!”.

Something else interesting here – the site where the Wilmking and JAcoby 1989 sites overlap is Sheenjek River. Jacoby archived a version going back to the 12th century, but Mann used an obsolete grey version that started later. Mann made an unjustified and unreported extension of the Gaspe series to get it into the AD1400 network of MBH98. In our first study in 2003, when we used archived versions of data, we used the Sheenjek River series instead of Gaspe. Mann squealed like a stuck pig about this. But when you look at the North American map using the criteria of Wilson and Pisaric, it’s completely impossible to justify use of Gaspe instead of Sheenjek River. It’s too bad that the dendrochronologists were suffering from the silence of the lambs.

Could it be that weather patterns in the artic are rather chaotic and the current warming trend causes changes thet slow tree growth in the particular area? Perhapse a warmer artic gets less moisture slowing tree growth?

Here’s the average for the following 8 site chronologies (my emulation of a Jacoby-type chronology for 7 sites with unarchived chronologies) at longitudinal treelines:
“ak085” “ak047” “ak048” “ak049” “ak050” “ak051” “ak052” “ak053” “cana209”

Now with respect to the sites used here. They are primarily sites sampled by Wilmking from the Brooks Range. Rob Wilson said on another thread:

This whole blog started with me criticising Steve M about averaging 14 sites without expressing where the sites came from and why they were originally sampled. Your 30 chronology mean is no different I am afraid. Not all the data are from the Jacoby group. Many of these series are from Wilmking. Again, I do not know the details of the individual sites, but Wilmking is a dendroecologist and has been study the divergence’ issue in recent years with respect to positive and negative responder trees.

So not all the sites are from the Jacoby group. However, if the “Jacoby” signal is robust, it should be observable by other sampling groups and any failure of other groups to obtain similar results to Jacoby calls into question the prudence of relying on results of one group only. My understanding of the Wilmking articles is that he discussed positive and negative responders because of the failure of the chronologies to repond linearly ot higher temperatures, not because they were a desideratum. His objective was clearly to sample temperature-sensitive locations.

How do you know that their reference to “treeline” is meant to refer to the latitudinal or altitudinal “treeline proper”, rather than the “species line” for the sampled species ?

The “species lines” of most tree species will be lower and farther south than the “treeline proper” formed at the boundary of the most cold-sensitive species. I haven’t read the papers, but the term “treeline” is ambiguous. Was the term well-defined ? Wikipedia had a link to the Online Glossary of Dendrochronology http://www01.wsl.ch/glossary/search-en.html , which has under the reference
for “tree limit”:

Alternative forms: = species line

Limit beyond which no individual trees occur, for example northernmost trees at the northern timber line.

the reference
for “timber line” states:

Alternative forms: ‘‰ˆ forest line

Limit of the closed forest. (‘‰ˆ Hustich 1983)

Considering that “the tree-line is a complicated ecological concept” (Hustich 1983) and in the absence of a standardised terminology of tree-line in most languages, we recommend that authors define explicitely any of the existing variants they use in their own articles.

The “timber line” definition has a graphic that can be accessed here with the following quote:

Tree-line and forest-line terminology:
1. timber line (synonyms : the economical or the generative forest-line);
2. physiognomical or vegetative forest-line, i.e. the limit of the closed forest regardless of its reproductive capacity;
3. tree-line proper, i.e. the polar, maritime or altitudinal tree-line;
4. the species-line, i.e. the absolute polar, maritime or altitudinal limit of the tree species in question, as seedling, krummholz, thickets, etc.);
5. historical tree-line (macrofossils and other evidence).
The belt between lines 2 and 5 is the forest-tundra ecotone.
(Hustich 1983)

One thing to take into consideraton is that if it’s not the actual treeline there will be competition with other trees. This will further confuse deciding if it’s a temperature signal or something else.

I don’t know, it strikes me that the “species line” would be a better place to sample than the “treeline proper”. No matter what, there are going to be competition problems for young trees, and a given tree probably doesn’t care whether his neighbor blocking out the light and sucking up all the available water is pine or a fellow fir.

On the other hand, if you measure sitka spruce at the sitka spruce “species line” you’ll have some confidence that those spruce are temperature limited and not water limited – because there are cold tolerant trees happily growing for several hundred feet up the slope. The same can’t necessarily be said when you go to the altitudinal “treeline proper” and look for BCP.

it strikes me that if one is sampling at the tree line from a species that is not notably cold hrdy and extant at the altitudinal or latitudinal tree line proper, then one must be sampling from the species tree line. aIIRC, a fundamental tenet of dendrochronology is to sample trees living ear the extreme of the limiting condition.If one is sampling cedar, one wants trees near the limits for cedar, and the fact that pines are thriving under those conditions and have a treeline much higher is simply irrelevant.

Agreed; my thinking, as well. If you look at the Gaspe satellite photo Steve M posted, it’s pretty clear it’s in the middle of a forest – not obviously near an altitudinal or latitudinal “treeline proper”. The other possibility is that cores are randomly taken away from any line, species or other, but I favor your interpretation. I’m sure these dendro guys can walk an extra mile and find a tree line – it just not clear if data handling and interpretation are always what they should be.

Given the common confusion it seems Hustich [2nd block, #8, above] was giving good advice.

I was agreeing with your first point – that sampling is occuring at the species line.

I disagree with your second point Re: relevance to the extent that if you’re looking for temperature proxies, it doesn’t seem apparent to me that there’s an easy way to categorically distinguish the highest BCP tree at the “treeline proper” as a temp or precip. proxy. The highest elevation cedar, on the other hand, may be 1000 or more feet from the “tree line proper” – and the fact that there are a million or so trees of a species with somewhat thicker bark growing higher up the mountain gives me a little bit of comfort that those cedars would be temp proxies and not precip proxies. My B.S. was in biology, so you don’t have to point out the obvious weakness of making strong claims here, species obviously plays a strong role with, for example, coastal redwoods [which require morning precip / fog]

As an avid follower (courtesy of Steve M) of all this temperature reconstruction stuff, it’s become patently obvious that tree ring widths will NEVER be a reliable measure of past and future trends in mean global surface temperature. I know this is bad news for those dendros who are currently largely funded (because of the current hysteria over catasthrophic man-caused global warming) to find temperature signals in tree rings which support the IPCC case, but tough.

IMO, ‘updating the proxies’ (those based on dendrochronology at least) is just a sheer waste of taxpayers money. Even if they were updated we’d still kick off yet another round of cherry-picking and academic debate on the rights and wrongs of which statistical method is appropriate or not for analysing such data. Much to the dismay of the HT, Steve M has done an excellent job of showing just how useless tree ring growth measurements are as a means of measuring past temperature. As the HT are fond of saying its now time to ‘move on’ and ensure that our hard earned taxes are spent on looking at other more reliable indicators of past, current and future temperature trends (radisonde, MSU etc).

I am still puzzled by the logic and mathematics of separating out temperature and precipitation from tr width when there are main and interaction effects. The selection of sites seems to acknowledge the importance of both factors not in reality control for one of them. Am I missing something?

I think we need to go back to first principles and look at what is needed to get a good temperature proxy, at least in theory. One of the criteria which is of interest is that the trees involved be sparce so that things like shading, root competition, fire and disease could be minimized. Looking at trees in a forest just don’t work according to this theory. Remember we’re going to be looking at those trees which are hundreds of years old at a minimum. Even an old-growth forest is going to have major problems in separating a true temperature signal from all the other things which could have happened in the distant past.

I’m at least sympathetic to Kevin’s remarks. I’d be moderately surprised if a true temperature signal can be obtained from tree rings at all. But I’m virtually certain it won’t be found in a forest.

#15. Kevin, I don’t mind money being spent on collecting more tree rings. It’s not very much money. The tree ring information has the big advantage that it can be accurately dated. There are a couple of recent tree ring studies tghat I’ve cited very favorably – NAurzbaev et al 2004 and Miller et al 2007. So I wouldn’t be at all quick to dismiss.

Also I think that the statistical methodology can be improved. There are huge and interesting tree ring data sets. I’d like to get some good applied statistical post-docs interested in this data. I’ve done some of my own work on improving the methods, but it’s one of many loose ends.

The utility of tree-ring information would be enhanced greatly if dendro collectors recorded the altitude from which each sample came from. I asked Rob Wilson offline one time whether there was any such data set that I could analyse and he said that, to his knowledge, there was NO dendro data set in which the altitude of each sample had been recorded. Given that Rob trained as a geologist, I find it hard to believe that he wouldn’t record this information in his own sampling just because of his geological training, but it seems that this is the case.

Eyeballing isn’t a statistical analysis, but those graphs do show a dip in the late 60’s & then a rise ~1976 (the PDO shift), but it’s unremarkable compared to other cycles. And looks like a 1998 El Nino pulse.

Certainly, one can qualitatively say these sites are “more” temp sensitive than arid bristlecone sites in the Amer west (which had already been identified in the literature as not correlating w/temps).

The treeline on each side of Hudson Bay is distinctly different because of the influence of the Bay on the circumpolar vortex. On the west side it is a quite distinctive line that follows the 10°C summer isotherm. On the east side it is a very indistinct line with very large outliers of trees and very large inliers of tundra. The Cri Lake site was always considered problematic because of these conditions. Brinkman and Barry attempted to clarify the climatic differences between the west and east sides of the Bay as early as 1972 following the work of Bryson in trying to establish the main position of the Polar Front. Blasing and Fritts (1976 p. 58) note “…the overall reconstructions imply a more meridional circulation in the 19th century than in the first half of the the 20th century.”

just to clarify some things. my data/trees from the brooks range ARE from latitudinal treeline, which coincides there with altitudinal treeline.

using the data from the ITRDB as was done in the graph above is a little problematic. there you averaged i assume the chronologies. when i archived my data in the ITRDB, i only submitted raw ring width of individual trees. since all of my sites contain sub-populations of positive and negative responders as you have also mentioned elsewhere, it is not advisable to average them. you basically average out the signal.

Are they actually negative responders; i.e. growth slows down with higher temperature, or are they non responders; i.e. ring width not correlating with temperature? And if the former, are there also non-responders? Finally in any case, what physical factors would account for negative vs positive responders and has any research explored individual trees to see which applied in a particular case?

#23. Hi, Martin. you’ve provided evidence that trees are positive and negative responders. It’s one thing to be able to classify 20th century trees, but how do you go about classifying 11th century trees – since presumably the same thing would occur if there were prior warming periods?

Martin:
This is interesting that within, I assume, close proximity you have positive and negative responders from with in the same species. How do you decide which is which? Are there generalizable factors that can be expressed as a variable in an equation – even as a dummy variable?

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